Three-dimensional networks in the 1:2 organic salts 2,2'-biimidazolium bis(3-carboxy-4-hydroxybenzenesulfonate) and 2,2'-bibenzimidazolium bis(3-carboxy-4-hydroxybenzenesulfonate) trihydrate.

The two title compounds of 2,2'-biimidazole (Bim) with 5-sulfosalicylic acid (5-H(2)SSA) and 2,2'-bibenzimidazole (Bbim) with 5-H(2)SSA are 1:2 organic salts, viz. C(6)H(8)N(4)(2+).2C(7)H(5)O(6)S(-), (I), and C(14)H(12)N(4)(2+).2C(7)H(5)O(6)S(-).3H(2)O, (II). The cation of compound (I) lies on a centre of inversion, whereas that of (II) lies on a twofold axis. Whilst compound (I) is anhydrous, three water molecules are incorporated into the crystal structure of (II). The substitution of imidazole H atoms by other chemical groups may favour the incorporation of water molecules into the crystal structure. In both compounds, the component cations and anions adopt a homogeneous arrangement, forming alternating cation and anion layers which run parallel to the (001) plane in (I) and to the (100) plane in (II). By a combination of N-H...O, O-H...O and C-H...O hydrogen bonds, the ions in both compounds are linked into three-dimensional networks. In addition, pi-pi interactions are observed between symmetry-related benzene rings of Bbim(2+) cations in (II).